1. Field of Invention
The present invention relates to a light guide plate, surface light source device and liquid crystal display, in particular, to improvements of a light guide plate having a back face provided with a great number of micro-reflectors, a surface light source device employing the foresaid improved light guide plate and a liquid crystal display employing the surface light source device for illumination a liquid crystal display panel. The present invention is applied to liquid crystal displays for personal computers, car-navigation systems or portable telephones, being applied further to surface light source devices and light guide plates used therein.
2. Related Art
According to a well-known prior art, a surface light source device has a light guide plate which is supplied with light sideways and outputs the light through an emission face after introducing the light into the light guide plate and applying direction-conversion, being broadly employed for illuminating an LCD panel or other uses. Although rod-like fluorescent lamps (cold cathode tubes) have been used broadly as primary light sources, those using point-like light sources such as LEDs (Light Emitting Diodes) tend to be employed recently.
In such surface light source devices, the light introduced into a light guide plate is outputted from an emission face after being light-direction-converted. As known well, light-direction-conversion within a light guide plate and emission from an emission face are promoted by employing a light guide plate made of light scattering-guiding material, or by applying emission promoting processing such as making a back face or emission face light-diffusible.
However, as known well, such means causes the emitted light to be preferentially directed to much forward inclined directions (for example, about 60 degrees with respect to a frontal direction). Such greatly inclined output directions are much quite different from usually desired output directions, which are usually generally frontal directions or around them. According to a prior proposition to realize a direction-conversion capable of providing a preferential output direction which is desired, a great number of micro-reflectors are formed on a back face of a light guide plate.
The micro-reflectors on the back face of the light guide plate in accordance with the proposed art are shaped like a great number of micro-projections, which generate an inner propagation light proceeding toward an emission face by means of an innerface reflection of the projections. This inner propagation light is emitted from the emission face, becoming an output light. Here described is an example of arrangement comprising a light source device, which employs a light guide plate provided with micro-reflectors, for backlighting a liquid crystal display panel by referring to FIG. 1 to FIG. 4.
In the first place, FIG. 1a is a back side plan view of an outlined arrangement of a surface light source device employing a light guide plate provided with micro-reflectors for backlighting of a liquid crystal display panel, and FIG. 1b is a side view from the left side in FIG. 1a. FIGS. 2a and 2b illustrate an array example of micro-reflector 20 in the arrangement. In these illustrations, a light guide plate denoted by reference numeral 10 is made of a transparent material such as acrylic resin, polycarbonate (PC) or cycloolefin-type resin, a side end face of which provides an incidence face 12.
A rod-like primary light source (cold cathode tube) L1 is disposed along the incidence face 12 which is supplied with light from the primary light source. The light guide plate 10 has major faces 13 and 14 one of which provides an emission face 13. The other face (back face) 14 is provided with a great number of micro-reflectors 20 shaped like micro-projections.
A well-known liquid crystal display panel PL is disposed on the outside of the emission face 13 to provide a liquid crystal display of backlighting type. It is noted that the micro-reflectors 20 are not shown in FIG. 1a. Size values are merely examples, being indicated in mm.
The primary light source L1 emits light, which is introduced into the light guide plate 10 through the incidence face 12. An inner propagation light travels within the light guide plate 10 and undergoes direction-conversion on entering into micro-reflectors 20 through inner-reflections by inner faces of projections, with the result that light proceeding toward the emission face 13 is produced. Such inner reflection occurs twice generally as described later.
An example of arrangement of micro-reflectors 20 on the back face 14 of the light guide plate 10 is shown in FIGS. 2a and 2b. It is noted that the primary light L1 disposed along the incidence face 12 is a rod-like cold cathode tube having an emitting portion length of which is somewhat smaller than that of the incidence face 12. Both ends are electrode portions EL1 and EL2 which are incapable of emitting light. Such a design is adopted often in order to avoid the electrode portions EL1 and EL2 of both ends from protruding.
Micro-reflectors 20 are distributed on the back face 14 so that covering rate tends to increase according to an increasing distance from the incidence face 12.
Micro-reflectors 20 are arranged in corner area C and D located close to the electrode portions EL1 and EL2, respectively, at a specifically large covering rate. Such a covering rate distribution prevents brightness from varying depending on distance from the incidence face 12 and from being short in the corner areas. It is noted that xe2x80x9ccovering ratexe2x80x9d of micro-reflectors is defined as area occupied by micro-reflectors per unit area of a back face of a light guide plate.
Each micro-reflector 20 is shaped like a quadrangle-pyramid, projecting from a general plane representing the back face 14 (i.e. a plane formed by provisionally removing the micro-reflectors 20). Each micro-reflector 20 has a posture determined as to cause light approaching there to be inner-inputted efficiently and to be converted into an inner output light proceeding generally at right angles with respect to the emission face 13. Such processes are described with referring to FIGS. 3, 4a, 4b and 4c. 
FIG. 3 shows one of the micro-reflectors 20 with an illustration of direction conversion of an inner propagation light effected by the micro-reflector. In the illustration, the inner propagation light is represented by representative light beams P1 and P2. Beam P1 represents an inner propagation light which is reflected by the slope 21 and then by the slope 22 in order while beam P2 represents an inner propagation light which is reflected by the slope 22 and then by the slope 21 in order. Beams QI and Q2 represent inner output light beams produced from beams P1 and P2, respectively.
It is noted that a pair of beams P1 and P2 run in parallel with a main approaching direction of light which is inner-inputted in a corresponding micro-reflector 20. In FIG. 3, coordinate O-XYZ is a right-hand coordinate used to denote directions, Z-axis of which extends vertically to the emission face 13 (more precisely, a second general plane representing the emission face; in the same way, hereafter) and has a +Z-direction that corresponds to a xe2x80x9cfrontal directionxe2x80x9d.
X-axis is perpendicular to both Z-axis and the incidence face 12, having an orientation (plus-minus sign) such that +X-direction extends as to get farther from the incidence face 12. Y-axis runs at right angles with respect to both Z-axis and X-axis as to provide a right-hand rectangular Cartesian coordinate O-XYZ (having original O optionally positioned), extending in parallel with the incidence face 12.
For the sake of description in the instant specification, a rectangular Cartesian coordinate O-xyz, which is independent of coordinate O-XYZ, is defined for each micro-reflector. Defined are x-axis, y-axis and z-axis as follows.
In the first place, z-axis extends in the same direction (including orientation) as that of Z-axis, having +z-direction which corresponds to the xe2x80x9cfrontal directionxe2x80x9d. A projection of a main approaching direction (including orientation) of light to be inner-inputted into a corresponding micro-reflector onto the emission face gives a direction of x-axis which extends perpendicularly to z-axis. And y-axis runs at right angles with respect to both z-axis and x-axis as to provide a right-hand rectangular Cartesian coordinate O-xyz (having original O optionally positioned).
It should be noted that x-axis may extend in a different direction as compared with X-axis and y-axis may extend in a different direction as compared with Y-axis in general, although O-xyz accords with O-XYZ in the case of the micro-reflector illustrated in FIG. 3. For example, micro-reflectors arranged in the corner portions C and D shown in FIG. 2 give y-axes non-parallel with Y-axis and x-axes non-parallel with X-axis because projections of main approaching directions of inner input light to the micro-reflectors onto XY-plane are inclined with respect to X-axis.
As illustrated in FIG. 3, each micro-reflector 20 has a pair of slopes 21 and 22 located on a side farther from the incidence face 12, the slopes providing a first and second inner-reflection faces. Both slopes (inner-reflection faces; in the same way, hereafter) 21 and 22 meet each other to form a ridge 25. There are another pair of slopes 23 and 24 located on a side nearer to the incidence face 12, the slopes meeting each other to form a ridge 26.
After all, in this embodiment, each micro-reflector 20 like a quadrangle-pyramid is provided by four slopes. It is noted that foot lines (inter sections with a general plane representing the back face 14) of the micro-reflectors are shown by dotted lines.
Viewing from the standpoint of light propagating within the light guide plate 10, the micro-reflectors 20 provide dents inside. The dents include valleys formed by the slopes 21, 22 and valleys formed by the slopes 23, 24.
Representative light beams P1 and P2 approach the micro-reflector 20 from the incidence face 12 directly or after being reflected by the emission face 13 and/or back face 14. Beams P1 and P2 reach one of the slopes 21 and 22. Some light may be directed to the slope 21 or 22 after being inner-reflected by the slope 23 or 24.
Much of light reaching the slope 21 or 22 is inner-reflected by the slope 21 and then by the slope 22, or by the slope 22 and then by the slope 21, with the result that an inner propagation light proceeding toward the emission face 13 is produced. This light is emitted from the emission face 13 to provide output light Q1, Q2 of the light guide plate 10. Thus a pair of 21 and 22 of each micro-reflector 20 function as a conversion output portion which makes an inner-output light from an inner-inputted light by converting a proceeding direction of the inner-inputted light. It is noted that references Q1 and Q2 are used to denote emitted beams.
Some consideration is given to postures of micro-reflectors 20 as follows. FIGS. 4a, 4b and 4c illustrate from three directions how light representing beams P1 and P2 inner-inputted to a micro-reflector formed in a standard posture are converted into inner output light Q1 and Q2 proceeding toward a frontal direction. FIG. 4a gives an illustration viewed from +z-axis direction (the same as +Z-direction due to definition), FIG. 4b gives an illustration viewed from +y-axis direction (the same as +Y-direction in this case), and FIG. 4c gives an illustration viewed from +x-axis direction (the same as +X-direction in this case).
Referring to these illustrations, behaviour of the above-mentioned representing beams P1 and P2 is described again with the use of the coordinate o-xyz.
As shown in FIG. 4a, representing beams P1 and P2 have an approaching direction to a micro-reflector 20 and the approaching direction provides a projection onto xy-plane in a direction consistent with +x-direction. Representing beams P1 and P2 inputted to the micro-reflector 20 are, as easily understood specifically from FIGS. 4b and 4c, inner-reflected by the slopes 21 and 22 inclined with respect to every one of xy-plane, yz-plane and zx-plane, being converted into beams Q1 and Q2 directed toward +z-direction.
These beams Q1 and Q2 represent inner output light, being parallel to each other. Beams Q1 and Q2 are emitted from emission face 13 toward +z-direction.
In the instant specification, if such direction conversion is effected by each micro-reflector having a posture (as shown in FIGS. 4a, 4b and 4c), the posture is called xe2x80x9cstandard posturexe2x80x9d. Standard posture requires the following conditions (1), (2) and (3) to be satisfied at the same time.
Condition 1; A projection of an extending direction of a ridge 25 of a conversion output portion onto xy-plane accords with x-axis direction (See FIG. 4a specifically).
Condition 2; A bisectional plane, which bisects an angle made by a first and second inner-reflection faces 21 and 22 so that the ridge 25 extends on the bisectional plane, is perpendicular to xy-plane (See FIG. 4a specifically).
Condition 3; An inner input light inner-inputted to the micro-reflector from a main approaching direction (+x-direction) is converted into an inner output light proceeding toward +z-axis direction.
If a light guide plate has a back face provided with a great number of micro-reflectors 20 arranged in such standard posture and the light guide plate is used in a surface light source device, primary light supplied sideways is converted directly into inner output light directed to a generally frontal direction which is outputted at a high efficiency, bringing a merit with a simple structure.
However, in prior arts employing micro-reflectors in standard posture tends to cause the output light to have an excessive directivity, being suffered from a problem that a small deviation of viewing direction from a main emission direction (i.e. direction of Q1 and Q2) brings a sharp reduction in brightness (Narrow viewing angle).
In particular, a posture on z-axis is made fitting in with the above-mentioned Condition 1, there rises a drawback that viewing angle in zx-pane differs much from that in yz-pane and the latter (viewing angle in yz-pane) is very small.
FIGS. 5 and 6 are graphs to illustrate results of simulation calculation of angular characteristics of emission intensity in a case where a micro-reflector is used in standard posture. In both graphs, abscissa indicates angles (inclination angles) in zx-plane wherein plotting s with signxe2x88x92correspond to a nearer side to the incidence face and plotting with sign+corresponds to a farther side from the incidence face. Ordinate indicates angles (inclination angles) in yz-plane, wherein plotting with sign+corresponds to right-handed inclinations as viewed from the incidence face and plotting with signxe2x88x92corresponds to left-handed inclinations as viewed from the incidence face.
In FIGS. 5 and 6, light intensity after a well-known Cosine correction (correction of values in accordance with cosine of inclinations of a light measuring direction) is illustrated in five discrete-intensity-levels. FIG. 6 is a graph for three-dimensional indication prepared based on the graph of FIG. 5, wherein light intensity is indicated in discrete-density shades and three-dimensional iso-brightness curves, and height from abscissa-ordinate plane expresses brightness (light intensity) after Cosine correction.
A set of parameters r, s, t are used as required for indicating a posture of a micro-reflector with respect to standard posture. It is noted that direction-angles (degrees) around z-axis, x-axis and y-axis are expressed by r, s and t with respect to those of standard posture, respectively. Of course, standard posture corresponds to r=s=t=0.
It is understood from the graphs that a main output direction (corresponding to Q1, Q2) has an angle on abscissa (angle in zx-plane) about 0 degree and an angle on ordinate (angle in yz-plane) about 0 degree, namely, being directed to a generally frontal direction, and that viewing angle in yz-plane is much smaller than that in zx-plane.
In addition to such a problem of small viewing angle, fine unevenness in brightness rises. That is, since inner output light has a very strong directivity, although a very high brightness is obtained just above a micro-reflector, only a small quantity of inner-inputted light reaches parts deviated from positions just above any micro-reflector (correspond between micro-reflectors), leading to darkness. As a result, fine unevenness in brightness arises, leading to feeling of glaring.
As described later about an example for comparison, although it has been tried relaxing the problem by loosing the above condition 1 or other means, satisfactory results have not been obtained.
An object of the present invention is to overcome the above-described drawbacks of prior arts and to provide a light guide plate, surface light source device and liquid crystal display employing the device which have an expanded viewing angle in a right-and-left-width direction as viewed from an incidence face side and a reduced fine unevenness in brightness that corresponds to micro-reflector-formed-position/micro-reflector-not-formed-position.
The present invention solves the above problem by means of giving three-dimensional bias with respect to standard posture (i.e. rotation around z-axis, pitching around x-axis and rolling around y-axis) to a great number of micro-reflectors formed on a back face of a light guide plate.
In the first place, the present invention is applied to a light guide plate that is supplied with light from a primary light source and has an emission face provided by a major face, a back face provided by another major face opposite with the emission face and an incidence face for light input.
The back face is provided with a great number of protrusion-shaped micro-reflectors for light-proceeding-direction conversion. Each of the micro-reflectors includes a conversion output portion that includes a ridge portion, a first inner-reflection face and a second inner-reflection face, the first and second inner-reflection faces extending on both sides of the ridge portion and being inclined with respect to a first representative plane representing the back face, respectively.
The ridge, the first inner-reflection face and the second inner-reflection face provide a valley inside the micro-reflector.
The valleys are configured as to get narrower and shallower according to an increasing distance from the incidence face. This causes an inner input light reaching the valleys to be reflected one of the first and second inner reflection faces and then the other of the first and second inner reflection faces and producing an inner output light proceeding toward the emission face.
And each micro-reflector is required to have a posture that satisfy conditions according to features of the present invention. To describe the conditions, xe2x80x9cx-axis, y-axis and z-axisxe2x80x9d are defined according to the following (1) to (3) and xe2x80x9cstandard posturexe2x80x9d is defined according to the following (41):
(1) x-axis is a projection of a main approaching direction of the inner input light onto a second representative plane representing the emission face;
(2); z-axis is an axis extending from the back face to the emission face as to be perpendicular to the emission face;
(3) y-axis is an axis extending perpendicularly to both x-axis and z-axis as to form a right-hand coordinate system in combination with x-axis and z-axis;
(4) xe2x80x9cstandard posturexe2x80x9d of a micro-reflector is defined as a posture such that satisfies the following Condition 1 to Condition 3 (the same as those forementioned) at the same time.
Condition 1; a projection of an extending direction of the ridge of the conversion output portion onto xy-plane accords with x-axis direction.
Conditions 2; a bisectional plane, which bisects an angle made by the first and second inner-reflection faces so that the ridge extends on the bisectional plane, is perpendicular to xy-plane.
Condition 3; an inner input light inner-inputted to the micro-reflector from a main approaching direction is converted into an inner output light proceeding in z-axis direction.
A posture of micro-reflector in accordance of a feature of the present invention is expressed as follows by using these definitions.
Each of the micro-reflectors has a posture which is substantially deviated from standard posture around x-axis, y-axis and z-axis. xe2x80x9cDeviationsxe2x80x9d of posture are preferably small angles.
Practical deviations of posture are, for instance, about 3 degrees to about 10 degrees around z-axis, 5 degrees to 15 degrees around x-axis and 1 degrees to 5 degrees around y-axis. Concrete examples are described later in Embodiments.
Such three-dimensional posture inclination causes an inner output light produced by each micro-reflector to have an angular expansion, in particular, in yz-plane. Accordingly, a natural viewing angle having a reduced directional bias is obtained. Further saying, angularly expanded proceeding directions are given to an inner output light produced by each micro-reflector, leading to an increased inner-incidence range (area) to an emission face. As a result, a reduced fine unevenness in brightness that corresponds to micro-reflector-formed-position/micro-reflector-not-formed-position is provided.
In the next place, the present invention is applied to a surface light source device having a primary light source and a light guide plate supplied with light from the primary light source. The light guide plate is one improved as above. A liquid crystal display in accordance with the present invention is provided by arranging the surface light source device for illuminating a liquid crystal display panel. The features of the light guide plate improved according to the present invention are inherited to the surface light source device and liquid crystal display.
That is, obtained are surface light source devices and liquid crystal displays which show viewing angles free from directional biasing to a specific direction. And the surface light source devices have a relaxed fine unevenness in brightness that corresponds to micro-reflector-formed-position/micro-reflector-not-formed-position, causing the result liquid crystal displays employing the surface light source devices to show an improved displaying quality.